4. Image file size must not exceed 2 megabytes (2,048kb), and the filename should contain only the characters A-Z and 0-9. Other non-standard ASCII characters, such as parenthesis, the # symbol, etc. are incompatible with our filesystem.

Charles has correctly recited the written rules, but as your image satisfies all of them, I suppose that isn't particularly helpful. And especially for a person approaching 1800 RP images.

I don't happen to see much in the way of pixellation in the sky, but it wouldn't hurt to a) sharpen more (not mentioned by the screener but I think it needs it), b) gaussian blur the sky a bit, and c) when you reupload, it is always a good thing to tell the screener what has changed.